NY 123 begins at the New York–Connecticut state line in the town of Lewisboro, as a continuation of CT 123 (Smith Ridge Road).

After crossing the state line, NY 123 is immediately within the hamlet of Vista, passing a couple businesses and residences as Smith Ridge Road.

A short distance later, NY 123 intersects with West Road, where it turns northward into a large commercial district before returning to the wooded residential backdrop.

After making a dart to the northwest, the route intersects with the western terminus of Spring Hill Lane, which connects to the former alignment of CT 35A after crossing the state line.

[3] NY 123 was first designated in the 1930 renumbering of state highways in New York as a continuation of the road leading north from Norwalk, Connecticut.